Heterodyne optical interferometers generally use input light beams that include two components of differing frequencies f1 and f2. One type of heterodyne interferometer uses one component (e.g., of frequency f1) in a measurement beam that is reflected from the object being measured and uses the other component (e.g., of frequency f2) in a reference beam. Analysis of the measurement and reference beams (after reflection) can determine a beat frequency that is equal to the frequency difference f1−f2 plus a Doppler shift Δf caused by reflection of the measurement beam from a moving object. The Doppler shift Δf, which may be small relative to the frequency f1 of the measurement beam, can be a significant fraction of the frequency difference f1−f2 and therefore can be precisely determined from the measured beat frequency. However, for an accurate indication of a Doppler shift Δf, the frequency difference f1−f2 in the input heterodyne beam should be greater than the maximum Doppler shift Δf encountered during measurements.
A Zeeman-split laser can provide an input light beam for a heterodyne interferometer. In a Zeeman-split laser, a magnetic field applied axially to a laser cavity creates a difference in the energies of emissions of left and right circularly polarized photons. As a result, the Zeeman-split laser has a gain curve for left circularly polarized light that differs from its gain curve for right circularly polarized light. If the laser cavity has an appropriate length, the Zeeman-split laser thus emits a beam including left circularly polarized light having one frequency and right circularly polarized light having another frequency. Unfortunately, the maximum frequency difference that can be practically created in a Zeeman-split laser is generally less than about 10 MHz, which may be smaller than the Doppler shift caused by reflection from a fast moving object.
An acousto-optic modulator (AOM) can be used with a laser (Zeeman-split or otherwise) to create or increase the frequency difference between the components beams. However, the robustness inherent of current AOM designs and required beam steering optics limit the usefulness of AOMs for creating high power heterodyne beams for precision interferometers.
A high power heterodyne input light beam is particularly important in an interferometer system where available space is limited but simultaneous measurements along many distinct axes are needed. In integrated circuit processing equipment, for example, a single heterodyne beam source may need to provide a heterodyne beam with sufficient power to simultaneously operate up to thirty or more interferometers. A beam source is thus needed that is capable of generating a heterodyne beam with a desired frequency difference and a power level suitable for multiple-axis interferometers.